There are many different types of protection mechanism that may be used in an optical communications network. Such protection mechanisms aim to provide protection in the event of failures of paths, links or devices within the network so that disruptions to communications services or loss of traffic are minimised.
It is known to provide a Sub-Network Connection Protection (SNCP) mechanism in a network operating a Synchronous Digital Hierarchy (SDH) protocol. Such a protection mechanism may use 1+1 protection where traffic in the network is actively transmitted from a source node to a sink node on both a worker path and a main protection path, and a selector device at the sink node is responsible for deciding which path to use. Alternatively 1:1 protection may be used where the traffic is only transmitted on the working path but a main protection path is allocated and used for sending traffic when a failure of the worker path occurs. Such 1+1 or 1:1 protection may be implemented as revertive protection or non-revertive protection. Revertive protection requires that traffic is switched to a protection path when the worker path fails, and then back to a worker path when the worker is available for use. Non-revertive protection requires that traffic stays on the protection path even when the worker path is available for use.
It is also known to provide nested protection to a worker path whereby a portion of the worker path may be further protected with a nested protection path. If the portion of the worker path fails then the nested protection path can be used for traffic. Such network protection provides protection for so-called “firm failures” which may be a permanent and complete failure of a path for longer than approximately 100 ms referred to by the International Telecoms Union standard ITU-T G.841.
In certain circumstances switching to the nested protection path can also trigger switching to the main protection path which may be unnecessary if switching to the nested protection path has restored the connection. It is known to define a Hold-Off (HF) time Tx at the sink node, according to the ITU-T G.841 standard, to prevent switching to the main protection path in the situation when switching to the nested protection path is an effective solution. The HF time is a period of time, typically between 50 ms and 2 s, to allow the nested protection path to restore the connection. If the nested protection path has not restored the connection at the end of the HF time then the main protection path is used for traffic. The HF time can be used to avoid unwanted protection switching events in the case of failures that are “short time” failures i.e. less than approximately 50 ms as referred to by the ITU-T G.841 standard.
When implementing revertive protection it is also known to define a Wait-to-Restore time (WTR) Δt, according to the ITU-T G.841 standard, such that once the traffic is switched to the protection path the selector at the sink node waits before switching back to the working path. The selector will only switch to the worker path if it is without failures during the whole of the wait-to-restore time. Typically the WTR time is of the order of 5-12 minutes.